1. Field of the Invention
The present invention relates to a process for preparing polytetrafluoroethylene powder.
2. Description of the Related Art
Polytetrafluoroethylene including a copolymer of tetrafluoroethylene and 1% by weight or less of a comonomer (hereinafter referred to as "PTFE") is blended with other thermoplastic resins, thermosetting resins, elastomers, paints, inks, greases, oils and the like to improve their properties by using the good properties of PTFE such as heat resistance, flame retardance, non-tackiness, low friction properties, chemical resistance, etc.
PTFE powder, which is most widely used for blending, is a low molecular weight product which is referred to as a "wax" and which has a melting point of about 310.degree. C. to 330.degree. C. PTFE powder used for usual molding is a high molecular weight product having a melting point of higher than 330.degree. C.
From a view point of a particle shape of the additive wax powder, PTFE powder is classified into agglomerated powder of submicron order particles and densified powder which has once been molten. The agglomerated powder has an average particle size of 2 to 100 .mu.m and a specific surface area of 9 m.sup.2 /g or larger, while the densified powder has an average particle size of 2 to 50 .mu.m and a specific surface area of about 2 to 6 m.sup.2 /g.
As disclosed in Japanese Patent Publication Nos. 22043/1982 and 25275/1976, a process for preparing a wax of agglomerated PTFE powder comprises emulsion polymerizing tetrafluoroethylene in the presence of a chain transfer agent to obtain a low molecular weight PTFE, coagulating an aqueous dispersion of colloidal PTFE and drying coagulated PTFE to obtain powder.
Since the PTFE powder obtained by the above process comprises agglomerates of colloidal particles, it can be advantageously dispersed to submicron order particles, while it cannot exhibit inherent friction characteristics of PTFE when it is used as an additive for a material in which a particle size of 5 to 20 .mu.m is required, for example, ink, since the agglomerates are broken.
When such PTFE powder is blended in some engineering plastics, the particles are poorly dispersed.
In the above applications, in particular, the densified PTFE powder is used.
For the production of densified PTFE powder, there is known a process comprising pyrolysing high molecular weight PTFE to decrease a molecular weight (cf. Japanese Patent Publication Nos. 15506/1975 and 20970/1963). However, this process has some drawbacks. That is, since a raw material PTFE is expensive, scraps such as swarfs are often used, so that impurities such as foreign particles tend to be contained in the product. Since liquid and gas by-products are always generated in the pyrolysis step, this process is uneconomical. The formation of hydrogen fluoride, which is a cause of corrosion of a pyrolysis apparatus, is unavoidable. In addition, since a product of pyrolysis is a dense bulk, it should be comminuted to a desired particle size.
However, it is not easy to comminute once molten PTFE. To achieve fine comminution, the molecular weight of PTFE is decreased. But, decrease of the molecular weight increases an amount of volatile components uneconomically.
Other process for preparing densified PTFE powder comprises comminuting a molded article which has been once molten by irradiation according to a method for decomposing high molecular PTFE by the irradiation (cf. Japanese Patent Publication Nos. 25419/1977 and 48671/1974). However, this process is not necessarily advantageous in view of apparatuses and costs. Even by this process, the impurities such as foreign particles tend to be contained in the product when the scraps are used as a raw material of PTFE.
As a process for producing densified PTFE particles containing less impurities, there is known a process comprising heating the above agglomerated powder at a temperature higher than the melting point of PTFE to melt the agglomerated powder, cooling and comminuting the heated powder. Since the PTFE powder is heated at a temperature higher than the melting point of PTFE, a large amount of volatile materials including low molecular weight polymers are generated, and the process economy is poor. Since the densified bulks are also produced in this process, PTFE should have an extremely low molecular weight or the agglomerated powder should be comminuted by a special technique such as freeze-comminution so as to obtain fine powder.